This invention relates to a molten metal filter having a coating of carbon or carbon and a thermite material on its surfaces. In comparison with uncoated filters, the use of the coating enables metals with lower pouring temperatures to pass through the filter during the filtration operation. The advantages of lower pouring temperatures are economic advantages of less energy usage and better casting quality. Use of carbon with thermite is especially advantageous in lowering the pouring temperature required in filtration. Still more particularly, the coated molten metal filters are used in the molten metal path in assemblies for mold casting and in continuous casting equipment.
As used herein including in the claims, "carbon" means any carbon or carbon-containing material that can be coated on and/or embedded at or in the surface of the structure or body of a molten metal filter and that will readily dissolve in molten metal passing through the filter without releasing any significant amount of gas.
In the processing of molten metals, it had been found advantageous to filter the metal in the liquid state to remove inclusions. To filter metal as a liquid requires a filter with extraordinary properties. The filter must be able to withstand extreme thermal shock, be resistant to chemical corrosion, and be able to withstand mechanical stresses. The present molten metal filter art employs ceramic monoliths, the main components of which are usually sintered silicon carbide, magnesia, zirconia, alumina, and/or silica with modifiers as required.
Generally in the working of molten metals, reduced metals are heated to above their melting points, the level of which is referred to as superheat, and poured into castings either for purposes of storage or for molding into a product. During the pouring operation, prior to the casting, a ceramic filter has been introduced to entrap inclusions out of molten metal. It has been discovered by those knowledgeable in the molten metal casting art that excluding certain contaminants during casting provides solid metals with superior properties at reduced costs.
Certain molten metals, for example super alloys, stainless steels, steel alloys, cast irons, and nonferrous metals are heated to temperatures which test the very limits of the physical and chemical properties of the filter. That these limits are exceeded is evidenced by catastrophic failure of the filter during the pour. During a catastrophic failure the filter breaks into many pieces. The filter may undergo less than catastrophic failure and still be inoperable due to some other failure mechanism. For example, if the mechanical strength of the ceramic material is exceeded the filter can deform in the direction of flow.
Ceramic filters are subject to chemical corrosion. The molten metal slag can, by way of illustration, attack the silicon-oxygen bonds in silica and thereby weaken the structural integrity of the filter. This slag attack or dissolution is a cause of significant failures in filters.
Finally some problems in filtering molten metals can be directly associated with the freezing of the molten metal as it contacts the filter. Since the filter is at temperatures significantly less than the molten metal pour temperature, the initial molten metal which contacts the filter must impart heat to the filter. Since the filter draws heat from the metal, the part of the molten metal affected decreases in temperature which cause the metal to freeze. As the metal solidifies in the filter, the solid metal can block entirely or at least partially the filterability of the filter, or it will slow the rate of filtering in the initial stages of the pour, thus decreasing filter performance.